Metal-organic frameworks (MOFs) are crystalline microporous materials with very high surface areas and ordered regular monodisperse pores. MOF pores generally range from about 0.25 nm to about 2.5 nm in diameter and are within the micropore and small mesopore regime. Because of their high porosity, MOFs have utility as materials for gas storage, gas separation, drug transport, molecular separations, catalysis, and sensors. However, current methods for producing MOFs are largely incompatible with many of the proposed applications. This is because many applications for MOFs, such as membranes for gas separation, require the MOF to be deposited or grown as continuous and crack-free films or membranes, hence the MOF material must be directly crystallized on a surface. Yet MOFs are solid, insoluble materials that are difficult to process, and it has been very challenging to grow MOFs on surfaces to date.
One method of growing MOF thin films/membranes is secondary growth crystallization. Initially, a first layer of seed nanocrystals is deposited on a macroporous support (e.g., Al2O3), and subsequently the seed layer is used to grow a continuous layer under solvothermal conditions. In some cases, better attachment is observed by prior functionalization of the macroporous support. This method requires multiple steps, and requires reaction under solvothermal conditions.
A second method is the Langmuir-Blodgett layer-by-layer approach, which relies on a Langmuir-Blodgett apparatus to make two dimensional MOF sheets, which can be then bridged by pyridine-based linkers between the sheets to form a three dimensional structure. Since the construction of the three dimensional structure relies on rinsing and immersing in a pyridine solution, and the metal clusters have open coordination for sheet linker ligands, this approach is not general in scope.
A third method of growing MOFs is in-situ crystallization. Here, a crystalline layer of a given MOF is grown directly on the bare surface of the support (or on a chemically modified support) in a one-step, one pot solvothermal synthetic procedure. The method requires heating the reaction mixture to provide solvothermal conditions.
A fourth method of preparing MOFs is gel-layer synthesis. Here, MOF materials are crystallized within a viscous gel layer and then transformed into films. As with in-situ crystallization, the gel-layer synthesis method requires heating the reaction mixture to provide solvothermal conditions.
A fifth method of preparing MOFs is referred to as liquid-phase epitaxial growth. This method requires sequential dipping of a functionalized surface in a solution containing either ligand or metal precursor. The solution usually does not contain both ligand and metal precursor at the same time. It is similar to epitaxial methods. The method is inefficient because solvent wash between dipping steps is often necessary and excess solution is needed to ensure enough component will remain to react with the next dipping step. For some MOFs, the approach is time-consuming because crystal growth may be slow.
A sixth method of preparing MOFs is anodic electrodeposition. This method has been used for the deposition of Cu3(BTC)2 (also known as HKUST-1, BTC=1,3,5-benzenetricarboxylate). It involves the oxidation of Cu metal electrodes to provide Cu2+ ions, which react with H3BTC ligands in the electrolyte solution. Crystals of Cu3(BTC)2 are deposited on the Cu electrode. This method is limited because the deposition surface is continually being corroded to supply one of the main starting materials in the MOF synthesis. In addition, the metal contained in the MOF is limited to the type of metal electrode used as the anode (e.g., a Cu electrode can only give rise to Cu-based MOFs).
There is a need for improved methods of preparing MOFs that do not require multiple steps, that do not require maintaining solvothermal conditions, that do not require corrosion of the deposition surface to provide a material in the MOF synthesis, and that are not limited to only certain metal ions.